/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.reactive.result.method;

import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.locks.ReentrantReadWriteLock;

import reactor.core.publisher.Mono;

import org.springframework.aop.support.AopUtils;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.core.MethodIntrospector;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.web.cors.CorsConfiguration;
import org.springframework.web.cors.reactive.CorsUtils;
import org.springframework.web.method.HandlerMethod;
import org.springframework.web.reactive.HandlerMapping;
import org.springframework.web.reactive.handler.AbstractHandlerMapping;
import org.springframework.web.server.ServerWebExchange;

/**
 * Abstract base class for {@link HandlerMapping} implementations that define
 * a mapping between a request and a {@link HandlerMethod}.
 *
 * <p>For each registered handler method, a unique mapping is maintained with
 * subclasses defining the details of the mapping type {@code <T>}.
 *
 * @param <T> The mapping for a {@link HandlerMethod} containing the conditions
 *            needed to match the handler method to incoming request.
 * @author Rossen Stoyanchev
 * @author Brian Clozel
 * @since 5.0
 */
public abstract class AbstractHandlerMethodMapping<T> extends AbstractHandlerMapping implements InitializingBean {

    /**
     * Bean name prefix for target beans behind scoped proxies. Used to exclude those
     * targets from handler method detection, in favor of the corresponding proxies.
     * <p>We're not checking the autowire-candidate status here, which is how the
     * proxy target filtering problem is being handled at the autowiring level,
     * since autowire-candidate may have been turned to {@code false} for other
     * reasons, while still expecting the bean to be eligible for handler methods.
     * <p>Originally defined in {@link org.springframework.aop.scope.ScopedProxyUtils}
     * but duplicated here to avoid a hard dependency on the spring-aop module.
     */
    private static final String SCOPED_TARGET_NAME_PREFIX = "scopedTarget.";

    /**
     * HandlerMethod to return on a pre-flight request match when the request
     * mappings are more nuanced than the access control headers.
     */
    private static final HandlerMethod PREFLIGHT_AMBIGUOUS_MATCH =
            new HandlerMethod(new PreFlightAmbiguousMatchHandler(),
                    ClassUtils.getMethod(PreFlightAmbiguousMatchHandler.class, "handle"));

    private static final CorsConfiguration ALLOW_CORS_CONFIG = new CorsConfiguration();

    static {
        ALLOW_CORS_CONFIG.addAllowedOrigin("*");
        ALLOW_CORS_CONFIG.addAllowedMethod("*");
        ALLOW_CORS_CONFIG.addAllowedHeader("*");
        ALLOW_CORS_CONFIG.setAllowCredentials(true);
    }


    private final MappingRegistry mappingRegistry = new MappingRegistry();


    // TODO: handlerMethodMappingNamingStrategy

    /**
     * Return a (read-only) map with all mappings and HandlerMethod's.
     */
    public Map<T, HandlerMethod> getHandlerMethods() {
        this.mappingRegistry.acquireReadLock();
        try {
            return Collections.unmodifiableMap(this.mappingRegistry.getMappings());
        } finally {
            this.mappingRegistry.releaseReadLock();
        }
    }

    /**
     * Return the internal mapping registry. Provided for testing purposes.
     */
    MappingRegistry getMappingRegistry() {
        return this.mappingRegistry;
    }

    /**
     * Register the given mapping.
     * <p>This method may be invoked at runtime after initialization has completed.
     *
     * @param mapping the mapping for the handler method
     * @param handler the handler
     * @param method  the method
     */
    public void registerMapping(T mapping, Object handler, Method method) {
        this.mappingRegistry.register(mapping, handler, method);
    }

    /**
     * Un-register the given mapping.
     * <p>This method may be invoked at runtime after initialization has completed.
     *
     * @param mapping the mapping to unregister
     */
    public void unregisterMapping(T mapping) {
        this.mappingRegistry.unregister(mapping);
    }


    // Handler method detection

    /**
     * Detects handler methods at initialization.
     */
    @Override
    public void afterPropertiesSet() {
        initHandlerMethods();
    }

    /**
     * Scan beans in the ApplicationContext, detect and register handler methods.
     *
     * @see #isHandler(Class)
     * @see #getMappingForMethod(Method, Class)
     * @see #handlerMethodsInitialized(Map)
     */
    protected void initHandlerMethods() {
        if (logger.isDebugEnabled()) {
            logger.debug("Looking for request mappings in application context: " + getApplicationContext());
        }
        String[] beanNames = obtainApplicationContext().getBeanNamesForType(Object.class);

        for (String beanName : beanNames) {
            if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
                Class<?> beanType = null;
                try {
                    beanType = obtainApplicationContext().getType(beanName);
                } catch (Throwable ex) {
                    // An unresolvable bean type, probably from a lazy bean - let's ignore it.
                    if (logger.isDebugEnabled()) {
                        logger.debug("Could not resolve target class for bean with name '" + beanName + "'", ex);
                    }
                }
                if (beanType != null && isHandler(beanType)) {
                    detectHandlerMethods(beanName);
                }
            }
        }
        handlerMethodsInitialized(getHandlerMethods());
    }

    /**
     * Look for handler methods in a handler.
     *
     * @param handler the bean name of a handler or a handler instance
     */
    protected void detectHandlerMethods(final Object handler) {
        Class<?> handlerType = (handler instanceof String ?
                obtainApplicationContext().getType((String) handler) : handler.getClass());

        if (handlerType != null) {
            final Class<?> userType = ClassUtils.getUserClass(handlerType);
            Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
                    (MethodIntrospector.MetadataLookup<T>) method -> getMappingForMethod(method, userType));
            if (logger.isDebugEnabled()) {
                logger.debug(methods.size() + " request handler methods found on " + userType + ": " + methods);
            }
            methods.forEach((key, mapping) -> {
                Method invocableMethod = AopUtils.selectInvocableMethod(key, userType);
                registerHandlerMethod(handler, invocableMethod, mapping);
            });
        }
    }

    /**
     * Register a handler method and its unique mapping. Invoked at startup for
     * each detected handler method.
     *
     * @param handler the bean name of the handler or the handler instance
     * @param method  the method to register
     * @param mapping the mapping conditions associated with the handler method
     * @throws IllegalStateException if another method was already registered
     *                               under the same mapping
     */
    protected void registerHandlerMethod(Object handler, Method method, T mapping) {
        this.mappingRegistry.register(mapping, handler, method);
    }

    /**
     * Create the HandlerMethod instance.
     *
     * @param handler either a bean name or an actual handler instance
     * @param method  the target method
     * @return the created HandlerMethod
     */
    protected HandlerMethod createHandlerMethod(Object handler, Method method) {
        HandlerMethod handlerMethod;
        if (handler instanceof String) {
            String beanName = (String) handler;
            handlerMethod = new HandlerMethod(beanName,
                    obtainApplicationContext().getAutowireCapableBeanFactory(), method);
        } else {
            handlerMethod = new HandlerMethod(handler, method);
        }
        return handlerMethod;
    }

    /**
     * Extract and return the CORS configuration for the mapping.
     */
    @Nullable
    protected CorsConfiguration initCorsConfiguration(Object handler, Method method, T mapping) {
        return null;
    }

    /**
     * Invoked after all handler methods have been detected.
     *
     * @param handlerMethods a read-only map with handler methods and mappings.
     */
    protected void handlerMethodsInitialized(Map<T, HandlerMethod> handlerMethods) {
    }


    // Handler method lookup

    /**
     * Look up a handler method for the given request.
     *
     * @param exchange the current exchange
     */
    @Override
    public Mono<HandlerMethod> getHandlerInternal(ServerWebExchange exchange) {
        if (logger.isDebugEnabled()) {
            logger.debug("Looking up handler method for path " +
                    exchange.getRequest().getPath().value());
        }
        this.mappingRegistry.acquireReadLock();
        try {
            HandlerMethod handlerMethod;
            try {
                handlerMethod = lookupHandlerMethod(exchange);
            } catch (Exception ex) {
                return Mono.error(ex);
            }
            if (logger.isDebugEnabled()) {
                if (handlerMethod != null) {
                    logger.debug("Returning handler method [" + handlerMethod + "]");
                } else {
                    logger.debug("Did not find handler method for " +
                            "[" + exchange.getRequest().getPath().value() + "]");
                }
            }
            if (handlerMethod != null) {
                handlerMethod = handlerMethod.createWithResolvedBean();
            }
            return Mono.justOrEmpty(handlerMethod);
        } finally {
            this.mappingRegistry.releaseReadLock();
        }
    }

    /**
     * Look up the best-matching handler method for the current request.
     * If multiple matches are found, the best match is selected.
     *
     * @param exchange the current exchange
     * @return the best-matching handler method, or {@code null} if no match
     * @see #handleMatch
     * @see #handleNoMatch
     */
    @Nullable
    protected HandlerMethod lookupHandlerMethod(ServerWebExchange exchange) throws Exception {
        List<Match> matches = new ArrayList<>();
        addMatchingMappings(this.mappingRegistry.getMappings().keySet(), matches, exchange);

        if (!matches.isEmpty()) {
            Comparator<Match> comparator = new MatchComparator(getMappingComparator(exchange));
            matches.sort(comparator);
            if (logger.isTraceEnabled()) {
                logger.trace("Found " + matches.size() + " matching mapping(s) for [" +
                        exchange.getRequest().getPath() + "] : " + matches);
            }
            Match bestMatch = matches.get(0);
            if (matches.size() > 1) {
                if (CorsUtils.isPreFlightRequest(exchange.getRequest())) {
                    return PREFLIGHT_AMBIGUOUS_MATCH;
                }
                Match secondBestMatch = matches.get(1);
                if (comparator.compare(bestMatch, secondBestMatch) == 0) {
                    Method m1 = bestMatch.handlerMethod.getMethod();
                    Method m2 = secondBestMatch.handlerMethod.getMethod();
                    throw new IllegalStateException("Ambiguous handler methods mapped for HTTP path '" +
                            exchange.getRequest().getPath() + "': {" + m1 + ", " + m2 + "}");
                }
            }
            handleMatch(bestMatch.mapping, bestMatch.handlerMethod, exchange);
            return bestMatch.handlerMethod;
        } else {
            return handleNoMatch(this.mappingRegistry.getMappings().keySet(), exchange);
        }
    }

    private void addMatchingMappings(Collection<T> mappings, List<Match> matches, ServerWebExchange exchange) {
        for (T mapping : mappings) {
            T match = getMatchingMapping(mapping, exchange);
            if (match != null) {
                matches.add(new Match(match, this.mappingRegistry.getMappings().get(mapping)));
            }
        }
    }

    /**
     * Invoked when a matching mapping is found.
     *
     * @param mapping       the matching mapping
     * @param handlerMethod the matching method
     * @param exchange      the current exchange
     */
    protected void handleMatch(T mapping, HandlerMethod handlerMethod, ServerWebExchange exchange) {
    }

    /**
     * Invoked when no matching mapping is not found.
     *
     * @param mappings all registered mappings
     * @param exchange the current exchange
     * @return an alternative HandlerMethod or {@code null}
     * @throws Exception provides details that can be translated into an error status code
     */
    @Nullable
    protected HandlerMethod handleNoMatch(Set<T> mappings, ServerWebExchange exchange) throws Exception {
        return null;
    }

    @Override
    protected CorsConfiguration getCorsConfiguration(Object handler, ServerWebExchange exchange) {
        CorsConfiguration corsConfig = super.getCorsConfiguration(handler, exchange);
        if (handler instanceof HandlerMethod) {
            HandlerMethod handlerMethod = (HandlerMethod) handler;
            if (handlerMethod.equals(PREFLIGHT_AMBIGUOUS_MATCH)) {
                return ALLOW_CORS_CONFIG;
            }
            CorsConfiguration methodConfig = this.mappingRegistry.getCorsConfiguration(handlerMethod);
            corsConfig = (corsConfig != null ? corsConfig.combine(methodConfig) : methodConfig);
        }
        return corsConfig;
    }


    // Abstract template methods

    /**
     * Whether the given type is a handler with handler methods.
     *
     * @param beanType the type of the bean being checked
     * @return "true" if this a handler type, "false" otherwise.
     */
    protected abstract boolean isHandler(Class<?> beanType);

    /**
     * Provide the mapping for a handler method. A method for which no
     * mapping can be provided is not a handler method.
     *
     * @param method      the method to provide a mapping for
     * @param handlerType the handler type, possibly a sub-type of the method's
     *                    declaring class
     * @return the mapping, or {@code null} if the method is not mapped
     */
    @Nullable
    protected abstract T getMappingForMethod(Method method, Class<?> handlerType);

    /**
     * Check if a mapping matches the current request and return a (potentially
     * new) mapping with conditions relevant to the current request.
     *
     * @param mapping  the mapping to get a match for
     * @param exchange the current exchange
     * @return the match, or {@code null} if the mapping doesn't match
     */
    @Nullable
    protected abstract T getMatchingMapping(T mapping, ServerWebExchange exchange);

    /**
     * Return a comparator for sorting matching mappings.
     * The returned comparator should sort 'better' matches higher.
     *
     * @param exchange the current exchange
     * @return the comparator (never {@code null})
     */
    protected abstract Comparator<T> getMappingComparator(ServerWebExchange exchange);


    /**
     * A registry that maintains all mappings to handler methods, exposing methods
     * to perform lookups and providing concurrent access.
     *
     * <p>Package-private for testing purposes.
     */
    class MappingRegistry {

        private final Map<T, MappingRegistration<T>> registry = new HashMap<>();

        private final Map<T, HandlerMethod> mappingLookup = new LinkedHashMap<>();

        private final Map<HandlerMethod, CorsConfiguration> corsLookup = new ConcurrentHashMap<>();

        private final ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();

        /**
         * Return all mappings and handler methods. Not thread-safe.
         *
         * @see #acquireReadLock()
         */
        public Map<T, HandlerMethod> getMappings() {
            return this.mappingLookup;
        }

        /**
         * Return CORS configuration. Thread-safe for concurrent use.
         */
        public CorsConfiguration getCorsConfiguration(HandlerMethod handlerMethod) {
            HandlerMethod original = handlerMethod.getResolvedFromHandlerMethod();
            return this.corsLookup.get(original != null ? original : handlerMethod);
        }

        /**
         * Acquire the read lock when using getMappings and getMappingsByUrl.
         */
        public void acquireReadLock() {
            this.readWriteLock.readLock().lock();
        }

        /**
         * Release the read lock after using getMappings and getMappingsByUrl.
         */
        public void releaseReadLock() {
            this.readWriteLock.readLock().unlock();
        }

        public void register(T mapping, Object handler, Method method) {
            this.readWriteLock.writeLock().lock();
            try {
                HandlerMethod handlerMethod = createHandlerMethod(handler, method);
                assertUniqueMethodMapping(handlerMethod, mapping);

                if (logger.isInfoEnabled()) {
                    logger.info("Mapped \"" + mapping + "\" onto " + handlerMethod);
                }
                this.mappingLookup.put(mapping, handlerMethod);

                CorsConfiguration corsConfig = initCorsConfiguration(handler, method, mapping);
                if (corsConfig != null) {
                    this.corsLookup.put(handlerMethod, corsConfig);
                }

                this.registry.put(mapping, new MappingRegistration<>(mapping, handlerMethod));
            } finally {
                this.readWriteLock.writeLock().unlock();
            }
        }

        private void assertUniqueMethodMapping(HandlerMethod newHandlerMethod, T mapping) {
            HandlerMethod handlerMethod = this.mappingLookup.get(mapping);
            if (handlerMethod != null && !handlerMethod.equals(newHandlerMethod)) {
                throw new IllegalStateException(
                        "Ambiguous mapping. Cannot map '" + newHandlerMethod.getBean() + "' method \n" +
                                newHandlerMethod + "\nto " + mapping + ": There is already '" +
                                handlerMethod.getBean() + "' bean method\n" + handlerMethod + " mapped.");
            }
        }

        public void unregister(T mapping) {
            this.readWriteLock.writeLock().lock();
            try {
                MappingRegistration<T> definition = this.registry.remove(mapping);
                if (definition == null) {
                    return;
                }

                this.mappingLookup.remove(definition.getMapping());
                this.corsLookup.remove(definition.getHandlerMethod());
            } finally {
                this.readWriteLock.writeLock().unlock();
            }
        }
    }


    private static class MappingRegistration<T> {

        private final T mapping;

        private final HandlerMethod handlerMethod;

        public MappingRegistration(T mapping, HandlerMethod handlerMethod) {
            Assert.notNull(mapping, "Mapping must not be null");
            Assert.notNull(handlerMethod, "HandlerMethod must not be null");
            this.mapping = mapping;
            this.handlerMethod = handlerMethod;
        }

        public T getMapping() {
            return this.mapping;
        }

        public HandlerMethod getHandlerMethod() {
            return this.handlerMethod;
        }

    }


    /**
     * A thin wrapper around a matched HandlerMethod and its mapping, for the purpose of
     * comparing the best match with a comparator in the context of the current request.
     */
    private class Match {

        private final T mapping;

        private final HandlerMethod handlerMethod;

        public Match(T mapping, HandlerMethod handlerMethod) {
            this.mapping = mapping;
            this.handlerMethod = handlerMethod;
        }

        @Override
        public String toString() {
            return this.mapping.toString();
        }
    }


    private class MatchComparator implements Comparator<Match> {

        private final Comparator<T> comparator;

        public MatchComparator(Comparator<T> comparator) {
            this.comparator = comparator;
        }

        @Override
        public int compare(Match match1, Match match2) {
            return this.comparator.compare(match1.mapping, match2.mapping);
        }
    }


    private static class PreFlightAmbiguousMatchHandler {

        @SuppressWarnings("unused")
        public void handle() {
            throw new UnsupportedOperationException("Not implemented");
        }
    }

}
